Development devices using two-component developer typically include a developer container in which developer is contained, a rotary developer bearer such as a development roller, and a developer conveyance member such as a conveyance screw provided in the developer container. The developer conveyance member supplies developer to the developer bearer while transporting the developer through a developer supply compartment (i.e., a developer supply path) inside the developer container in an axial direction of the developer bearer. Then, the developer bearer rotates and supplies the developer carried thereon to a development range facing an image bearer such as a photoreceptor. After having passed through the development range and toner therein has been consumed, the developer (hereinafter “used developer”) is collected either in the supply compartment (hereinafter “single-conveyance path method”) or a collection compartment separate from the supply compartment (hereinafter “supply-collection separation method”).
The single-conveyance path method has a drawback in that the concentration of toner in the developer in the supply compartment decreases downstream in a direction in which the developer is transported (hereinafter “developer conveyance direction”). Accordingly the concentration of toner in the developer supplied to the development range is uneven in the axial direction of the developer bearer (it is to be noted that hereinafter the terms “downstream” and “upstream” used in this specification mean those in the developer conveyance direction unless otherwise specified). Such unevenness in the toner concentration causes unevenness in image density of images formed on sheets of recording media and is undesirable. In particular, currently, images of higher printing ratio, such as photographs, are output more frequently than images of lower printing ratio, such as those include mainly text. Images of higher printing ratio consume more toner, with the result that the concentration of toner in the developer tends to become uneven, and the unevenness in image density resulting from the uneven toner concentration tends to be more visible.
To solve the problem described above, for example, JP-H11-184249-A employs a supply-collection separation method in which used developer is collected in the collection compartment separate from the supply compartment (hereinafter “a supply-collection separation type development device”). In the supply-collection separation method, the concentration of toner in the developer in the supply compartment can be kept substantially constant in the developer conveyance direction. Thus, the concentration of toner in the developer supplied to the development range can be kept uniform in the axial direction of the developer bearer.
Still, image density can become uneven in the axial direction of the developer bearer in supply-collection separation type development devices when the fluidity of the developer is reduced due to deterioration of the developer over time or from environmental factors.
In the supply-collection separation type development devices, because the developer is supplied to the developer bearer while being transported through the developer supply compartment, the amount of the developer flowing in the developer supply compartment decreases downstream. In other words, the amount of developer in the supply compartment is greater on the upstream side than on the downstream side in the developer conveyance direction. Where the amount of developer is greater, the mobility of the developer is lower.
Thus, in supply-collection separation type development devices, there is a portion where the mobility of the developer is lower in the developer conveyance direction, that is, in the axial direction of the developer bearer. In such a portion, the amount of the developer that passes through a regulation gap tends to drop.
To restrict the drop in the amount of developer that passes through the regulation gap, resulting from the decrease in the fluidity of the developer, the force for transporting the developer in the rotational direction of the developer bearer may be increased by increasing the magnetic force exerted by the magnetic field generator or abrading the surface of the developer bearer. However, such approaches can increase the stress on the developer, thus accelerating the degradation of the developer.